Question

A resistance $R$ is to be measured using a meter bridge. Student chooses the standard resistance $S$ to be $100 \,\Omega$. He finds the null point at $l_2 = 2.9 \,cm$. He is told to attempt to improve the accuracy. Which of the following is a useful way?

• A. He should measure $l_2$ more accurately
• B. He should change $S$ to $1000 \Omega$ and repeat the experiment
• C. He should change $S$ to $3 \Omega$ and repeat the experiment
• D. He should give up hope of a more accurate measurement with a meter bridge

Answer 1

Answer: (C)

As the meter bridge is balanced,
$\therefore \frac{R}{S} = \frac{l_1}{(100-l_1)}$,
or $R = \frac{Sl_1}{(100-l_1)}$
$ \frac{(100 \Omega)(2.9\,cm)}{(100 -2.9) cm} \approx 3 \Omega$
The accuracy of measuring $R$ can be improved if $S$ and $R$ are of the same order, i.e., $S$ should be changed to $3\Omega$